This invention generally relates to a bandpass filter for ultrahigh frequency (UHF) band, and more particularly, the present invention relates to such a bandpass filter having a sub transmission line.
The attenuation characteristics of a bandpass filter, referred to as BPF hereafter, for UHF band is basically determined by the number of stages, namely the number of resonators, included therein. In order to make the attenuation characteristic sharp, the number of stages may be increased. However, increasing in the stage number necessarily increases the loss at the transmission range. Therefore, it is necessary to make the unloaded Q of the resonators large. Since the unloaded Q of a resonator usually increases in proportion to the volume of the resonator, it can be said from the above that it is necessary to increase the number and the size of the resonators to provide a sharp attenuation characteristic when it is desired to maintain a low loss in the transmission range. This means that it is inevitable that a superior BPF tends to be bulky. As a method for resolving this problem, a technique of providing peak attenuation points in the rejection range is known. According to this technique, a sub transmission line is additionally provided as is described later. However, this conventional technique could only be applied to a filter having an even number of stages. Furthermore, the coupling degree of the conventional sub transmission line or branching circuit is difficult to adjust, while the peak attenuation points cannot be satisfactorily controlled with high repeatability.